textura, composición y contenido de materia orgánica de los sedimentos recientes de un lago tropical de México

Abstract

Laguna Escondida is a small deep lake (A=18.26 ha; z_m = 32.5 m) sorrounded by the tropical rain forest still remaining in the "Los Tuxtlas" region, in the Mexican state of Veracruz. The superficial sediments of the lake were studied in order to identify its texture, origin, composition and organic matter content. The texture analysis led us to recognize nine groups ranking from gravely-sands, at the inlet of the main affluent and at the center of the west shore, to clayey-silts in the deepest parts of the lake. The energy of this main affluent dissipates as the current penetrates into the lake and the materials carried by the river are deposited in a sedimentary plume of particles of decreasing size. Most of the lake's bottom is covered by allochthonous plant debris in several degrees of decomposition. These came mainly from the dense canopies of the forest surroinding the lake. Sediments are composed by allogenic minerals of volcanic origin (mainly silicates) and small quantities of endogenic (sponge spicules, diatom frustules and ostracods) and autigenic (iron oxides and philipsite) minerals, both in quantities of less than 5%. Allochthonous organic matter (OM) is extremely abundant; its proportion varies from 20 to 30% at the center of the lake but it reaches 80% in the northwestern shores. High concentrations of OM indicates that the detrital food-chain plays an important role in lake's metabolism. The nature and distribution of the sediments suggest that Laguna Escondida can be seen as a mixed system from a tropic perspective. Thus, it would be typified as a mesotrophic-polihumic lake. Finally, it is indicated that the cuting of the forest that occupies the lake surroundings will promote profound changes in the metabolism of this lake.